1. Field of the Invention
This invention relates to a process for the manufacture of molded articles from powder-form polymers of ethylene or copolymers of ethylene and .alpha.-olefins with 3 to 10 carbon atoms with minimum molecular weights of one million.
2. Discussion of the Prior Art
Homo- and copolymers of olefins, especially polyethylenes, with molecular weights over one million and densities between 0.92 and 0.96 g/cm.sup.3 have been known for some time. Due to their specific properties, for example good sliding characteristics, low abrasion, extreme toughness and excellent resistance to solvents and corrosive chemicals, molded articles from these polymers have proven to be valuable in various industrial fields. Due to their remarkable skin compatibility and physiological neutrality, high molecular polyolefins are suitable for the manufacture of prostheses and supports as well as for implant material in bone surgery.
Olefin polymers with molecular weights over one million exhibit excellent chemical and physical properties. There are, however, considerable difficulties in their processing. In contrast to the low molecular polyolefins, the high molecular products can only be processed using molds and screw-type or Ram extruders, due to their extremely high melt viscosity. After molding and extrusion processing, a semi-finished product is obtained, from which the desired molded articles have to be produced by machining.
A process for the manufacture of solid articles from ultra-high molecular polyethylene powder is described in German Offenlegungsschrift No. 2 425 396. The powderform starting material contains particles of average size less than 100.mu. and a distribution function of less than 0.80. The powder is molded to a solid preform at a pressure of at least 140 kg/cm.sup.2 and a temperature below the crystalline melting point. After the release of pressure, the preform is sintered at a temperature above the crystalline melting point. As the ultra-high molecular polyethylene usually contains particles above 100.mu., it is necessary with this method to reduce the starting material to the required particle size by appropriate means, after the polymerization process, e.g., by grinding. An additional disadvantage of the known method lies in the fact that the molded articles are not homogeneously plasticized and during the sintering process they expand, the extent of which depends on the temperature. This expansion is due to the fact that the preform is not sintered in a mold. As a result of this, the physical properties are not consistent and vary considerably. Moreover, a high dimensional stability of the preform is not guaranteed.